Ink or toner can be a significant cost over the lifetime of a printer or copier. Therefore, using less ink results in overall cost savings. In general, the amount of ink used contributes significantly to the cost of printing the page. If each printed page uses less ink, then that ink savings translates into direct cost savings. Also, environmental benefits may come in the paper recycling process where use of less ink can result in needing fewer toxic chemicals. Furthermore, the paper recycling process is a hydrodynamic process, and thus less ink on paper may result in less water usage and less power to pump the process.
While ink reduction is a desirable result, current color printers do not efficiently use ink. Methods for reducing ink usage includes draft mode technology, pre-halftoning technologies, and halftoning algorithms that reduce ink while maintaining image quality. With respect to the draft mode technology, some current printers provide a ‘draft’ option that reduces ink by reducing image quality. For example, a draft document may be printed by only printing alternate dots of an image, essentially halving the brightness of the 8 bit C, M, Y and K values before halftoning, or decreasing the saturation of an image before halftoning. Because draft printing reduces the quality of the document, this technique provides only limited usefulness in reducing the quantity of ink used by a color printer or copier.
Another set of technologies changes or limits the 8-bit data before the halftoning. See for example, U.S. Pat. No. 6,313,925, entitled “System, Method, and Program for Saving Toner/Ink in a Color Printer Without Sacrificing Image Quality,” issued Nov. 6, 2001 to Decker et al. Special undercolor removal functions are sometimes used.
Previous work has modified error diffusion to compensate for dot gain. See, for example, Henry Kang, Digital Color Halftoning, SPIE Press, 2001 and Joseph Shu, “Error Diffusion With Ink Reduction for High Quality and High Resolution Ink Jet Printing,” IEEE International Conference on Image Processing, 1995. Shu refers to his algorithm as “Error diffusion with ink reduction.” He states that he compensates for the dot gain in order to reduce ink to prevent ink overflow on the ink jet media and to reduce worm-artifacts. Shu's work also appears in patent form. See U.S. Pat. No. 5,592,592, entitled “Method and Apparatus for Minimizing Artifacts in Images Produced by Error Diffusion Halftoning Utilizing Ink Reduction Processing,” issued Jan. 7, 1997 to Shu.
Another relevant body of technologies is the set of halftoning algorithms that do not allow K overprinting. There are many dither methods. For example, see U.S. Pat. No. 5,473,446, entitled “Color Digital Halftoning Using Black and Secondary Color Replacement and Color Vector Dithering,” issued Dec. 5, 1995 to Perumal, et al.
U.S. Pat. No. 5,333,243, entitled “Method for Forming Color Images Using a Hue-Plus-Gray Color Model and Error Diffusion,” issued Jul. 26, 1994 to Best et al., describes a novel error diffusion method in which only one color (R, G, B, C, M, Y, K, W) is printed at a time. This disallows the possibility of printing K on top of a color. The system decides which color to print by noting which color has the current largest error. Error is passed as in normal error diffusion. Such methods require special undercolor removal or limitations on the input or else the error will overflow. For example, the CMYK input value (255, 0, 0, 50) would cause this method to have increasingly large errors.
In U.S. Pat. No. 5,402,245, entitled “Bi-level digital color printer system exhibiting improved undercolor removal and error diffusion procedures,” issued Mar. 28, 1995 to Motta et al., an error diffusion algorithm is described in which a decision is first made to print K or not based on whether the input k value is the largest input value and if it is greater than the error diffusion threshold. If it is decided to print K, then no other colors (C, M, Y) are considered. Again, special undercolor removal or limitations on the input are necessary to ensure that the error does not overflow. For example, the CMYK input color (50, 0, 0, 255) would cause this method to have increasingly large errors.
Work by Motta and Dispoto includes the idea of a mapping between the quantizer and the output halftone signal. In their work the error diffusion steps of quantizing, error calculation, and error distribution happen in one space (such as CIELab, or RGB space) and then the output from the quantizer is mapped to an appropriate color in the printing device space. See U.S. Pat. No. 5,621,545, entitled “Image production using color error diffusion,” issued Apr. 15, 1997 to Motta et al. However, this patent only describes mapping to CMY space.